Correlation Field Splitting in a Two‐Dimensional Phase: CO₂Adsorbed on the NaCl(100) Cleavage Plane

The infrared absorption of CO 2 adsorbed on the NaCl(100) cleavage plane was studied by means of Fourier transform spectroscopy. At monolayer coverage the v 3 vibration reveals a doublet structure, while for the v 2 vibration four absorption bands are observed. The dependence of the v 3 doublet on the isotope composition of the adsorbate excludes different adsorption sites and demonstrates that correlation field splitting is responsible for the observed spectral features. The two‐fold degeneracy of the bending mode, present in the gas, is lifted due to the adsorption site symmetry C 1 , and each of the two v 2 modes splits into a doublet due to the correlation field. From polarization measurements the tilt angle between the CO 2 molecular axis and the NaCl(100) surface was determined to be ϑ = (27 ± 5)°. A herringbone model for the (2 × 1) adsorbate structure with the two‐dimensional space group pg can explain the observed spectral multiplets.